Activation response and functional connectivity change in rat cortex after bilateral transcranial direct current stimulation-An exploratory study.

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique implicated as a promising adjunct therapy to improve motor function through the neuromodulation of brain networks. Particularly bilateral tDCS, which affects both hemispheres, may yield stronger effects on motor learning than unilateral stimulation. Therefore, the aim of this exploratory study was to develop an experimental model for simultaneous magnetic resonance imaging (MRI) and bilateral tDCS in rats, to measure instant and resultant effects of tDCS on network activity and connectivity. Naïve, male Sprague-Dawley rats were divided into a tDCS (n = 7) and sham stimulation group (n = 6). Functional MRI data were collected during concurrent bilateral tDCS over the sensorimotor cortex, while resting-state functional MRI and perfusion MRI were acquired directly before and after stimulation. Bilateral tDCS induced a hemodynamic activation response, reflected by a bilateral increase in blood oxygenation level-dependent signal in different cortical areas, including the sensorimotor regions. Resting-state functional connectivity within the cortical sensorimotor network decreased after a first stimulation session but increased after a second session, suggesting an interaction between multiple tDCS sessions. Perfusion MRI revealed no significant changes in cerebral blood flow after tDCS. Our exploratory study demonstrates successful application of an MRI-compatible bilateral tDCS setup in an animal model. Our results indicate that bilateral tDCS can locally modulate neuronal activity and connectivity, which may underlie its therapeutic potential.

In vivo imaging of cerebral glucose metabolism informs on subacute to chronic post-stroke tissue status - A pilot study combining PET and deuterium metabolic imaging.

Recanalization therapy after acute ischemic stroke enables restoration of cerebral perfusion. However, a significant subset of patients has poor outcome, which may be caused by disruption of cerebral energy metabolism. To assess changes in glucose metabolism subacutely and chronically after recanalization, we applied two complementary imaging techniques, fluorodeoxyglucose (FDG) positron emission tomography (PET) and deuterium (2H) metabolic imaging (DMI), after 60-minute transient middle cerebral artery occlusion (tMCAO) in C57BL/6 mice. Glucose uptake, measured with FDG PET, was reduced at 48 hours after tMCAO and returned to baseline value after 11 days. DMI revealed effective glucose supply as well as elevated lactate production and reduced glutamate/glutamine synthesis in the lesion area at 48 hours post-tMCAO, of which the extent was dependent on stroke severity. A further decrease in oxidative metabolism was evident after 11 days. Immunohistochemistry revealed significant glial activation in and around the lesion, which may play a role in the observed metabolic profiles. Our findings indicate that imaging (altered) active glucose metabolism in and around reperfused stroke lesions can provide substantial information on (secondary) pathophysiological changes in post-ischemic brain tissue.

Direct B0 field monitoring and real-time B0 field updating in the human breast at 7 Tesla.

Large dynamic fluctuations of the static magnetic field (B(0)) are observed in the human body during MR scanning, compromising image quality and detection sensitivity in several MR imaging and spectroscopy sequences. Partially, these dynamic B(0) fluctuations are due to physiological motion such as breathing, but other sources of temporal B(0) field fluctuations are also present in the MR system (e.g., eddy currents). Especially at ultrahigh field (≥7 T), the increased susceptibility effects lead to large B(0) field variations over time. Direct measurement and correction of these temporal field variations of up to 70 Hz will lead to a significant reduction of artifacts and improved measurement stability/reproducibility. For direct measurement of the temporally changing B(0) field, a simple field probe was developed, that was placed in proximity to the tissue of interest. In this work, it is shown how such a field probe system can be used to monitor temporal B(0) field variations in the human body during MRI and magnetic resonance spectroscopy. Furthermore, it is shown how the acquired temporal B(0) field information can drive a dynamic shim module to directly correct the B(0) magnetic field in real time.